The present invention relates to an image reading device having an automatic focusing function.
In a camera-type image reading device, since the distance between the image reading device and an object varies, data related to the distance should be obtained in order to focus on the object.
In the field of photography in which a camera uses a film, an active and passive distance measuring method are well known. In the active method, an infrared light is emitted from the camera to the object, and by detecting the reflected infrared light, the distance is measured. In the passive method, by detecting a phase difference of an object image, or detecting a distribution of contrast information of the object, the distance or defocus amount is detected. In such a camera, an automatic focusing mechanism is provided, which moves a focusing lens in accordance with the detected object distance or the defocus amount so that an in-focus image is formed on a film plane.
Specifically, when the phase difference of the object image is detected, a pair of separator lenses are provided to form a pair of images to be compared with each other. When the contrast distribution is detected, a half mirror is used to split the light from the object, and the contrast distribution on planes which have different optical distances from a focusing lens.
In the image reading device using an image sensor, such as a CCD (Charge Coupled Device) to capture an object image, since the optical image is converted into a digital data which can be processed by a microprocessor or the like easily, it is preferable that the focus condition is detected with use of the image sensor. However, the conventional methods mentioned above cannot be applied to the image capturing system, and therefore, in order to employ the conventional focusing system, an extra optical system should be introduced in the image reading device.